In the fields of home electrical appliances, industries, and automobiles, motor drive apparatuses are employed in rotation speed control for fan, pump, compressor, and the like, torque assist devices for electric power steering and the like, and positioning control for conveyer and lift. A permanent magnet synchronous motor (which will be denoted as “PM motor” below), which is a small and high-efficiency AC motor, is widely employed in the motor drive apparatuses in these field. However, information on magnetic pole position of a rotor of the motor is required for driving the PM motor, and a position sensor such as resolver or hall IC therefor is essential. In recent years, there is widely used sensorless control for controlling a frequency or torque of a PM motor without the use of such a position sensor.
The realization of sensorless control enables cost for the position sensor (cost for sensor itself, cost for sensor wiring, and cost for sensor attachment/adjustment work) to be reduced, and the unnecessity of the sensor accordingly causes a merit that an apparatus can be downsized or can be used under deteriorated environments.
At present, the sensorless control for PM motor employs a system for directly detecting an inductive voltage (speed induced voltage) caused by rotation of a rotor and assuming it as rotor position information thereby to drive the PM motor, a position estimation technique for estimating and calculating a position of the rotor based on the mathematical models of a PM motor, and the like.
These are a system using a speed induced voltage in principle, and are difficult to apply in an area where the speed induced voltage is low due to stop or in a low-speed period. Thus, these techniques are applied mainly in middle- and high-speed ranges, and open loop control such as V/F constant control is employed in a low speed range. In the case of open loop control, motor-generated torque cannot be freely controlled, and thus controllability in the low speed range is deteriorated and the efficiency is also deteriorated.
There has been already proposed a system for acquiring rotor position information in a low speed range against the above.
In PTL 1, a pulse voltage is applied to two phases in a three-phase PM motor and an open voltage of the non-conducted remaining phase is detected thereby to acquire position information from the voltage. An induced voltage in the open phase is generated depending on a position of the rotor of the PM motor, and can be used to estimate a position of the rotor. The induced voltage is generated by a slight change in inductance in the motor due to a relationship between a permanent magnetic flux attached on the rotor of the PM motor and a conductive current by the pulse voltage, and can be observed also in the stop state. This is denoted as “magnetic saturation induced voltage.”
Further, in the system, 120-degree conductive drive is essential to select and conduct two phases out of the three phases in order to observe an induced voltage of the non-conducted phase (open phase). The conducted phases need to be switched depending on a position of the rotor in order to perform position-sensorless drive. The “magnetic saturation induced voltage” caused in the open phase is used for switching the conducted phases.
The magnetic saturation induced voltage monotonically increases or decreases depending on a position of the rotor. Thus, in PTL 1, position sensorless control is performed to switch to a next conducted-phase when a “threshold” is provided for the induced voltage of the open phase and the magnetic saturation induced voltage reaches the threshold. At this time, the “threshold” is a remarkably important setting element. The threshold slightly varies per motor or per phase wiring of the motor, and needs to be appropriately set. PTL 2 describes therein a method for automatically performing an adjustment work therefor per motor.
To the contrary of the method described in PTL 1, in PTL 2, an automatic adjustment routine is previously performed on a threshold, and thus a worker does not need to manually make the adjustment, thereby saving the system startup work.
The published patents assume the 120-degree conductive drive, but a sinusoidal drive method has been already reported. In PTLs 3 and 4, a PM motor employs a three-phase stator wiring in Y connection thereby to observe a connection point potential of the three-phase wiring in Y connection (which is denoted as neutral point potential), thereby estimating a position of the rotor.
An open phase does not need to be observed unlike in PTL 1, and thus three phases can be conducted at the same time, thereby driving a PM motor at an ideal sinusoidal current. However, it is essential to detect a neutral point potential.
PTL 3 describes therein a voltage pulse insertion method for observing a neutral point potential. Further, PTL 4 describes that a neutral point potential is observed in association with a PWM pulse for pulse width modulation by use of a voltage applied to an inverter for driving the PM motor, thereby instantaneously estimating and calculating a position of the rotor.